This invention is generally directed to new colored toners, and more specifically positively charged colored developer compositions, wherein a single common carrier is employed, which developers are useful for obtaining images of different colors.
The formation and development of images in an electrophotographic system, and more specifically a xerographic system is well-known as described, for example, in U.S. Pat. No. 2,297,691. In such systems, several methods are used for applying the electroscopic particles or toner to the latent electrostatic image to be developed, such methods including cascade development, U.S. Pat. No. 2,618,552; magnetic brush development, U.S. Pat. No. 2,874,063; powder cloud development, U.S. Pat. No. 2,221,776; touchdown development, U.S. Pat. No. 3,166,432 and the like. Generally, the toners used in these types of systems result in the production of black images.
Recently there has been developed processes and materials for use in the formation of color images. Electrophotograhic color systems are generally based on trichromatic color synthesis, such as the substractive color formation types. Accordingly, in electrophotographic color systems, toner or developing particles of at least three different colors are employed to synthesize a desired color. Generally, at least three color separation images are formed and the combined images register with each other to form a colored reproduction of a full colored original. In color xerography as described, for example, in Dessauer U.S. Pat. No. 2,972,374, at least three latent electrostatic images are formed by exposing a xerographic plate to different optical color separation images. Each of the latent electrostatic images is developed with a different color toner and subsequently the three toner images are combined to form the final full color image. This combination of three color toner images is generally made on a copy sheet such as paper to which the toner images are permanently affixed. One of the most common techniques for fixing these toner images to the paper copy sheet is by employing a resin toner which includes a colorant, and heat fixing the toner images to this copy sheet. Images may also be fixed by other techniques such as, for example, subjecting them to a solvent vapor.
In one known process an electrostatic latent image is exposed through a green filter to an imagewise projection of a color image to form an electrostatic latent image on the photoreceptor. This electrostatic latent image is then developed with the complimentary magenta color toner to form a magenta colored image corresponding to said electrostatic latent image, and transferred in register to an image receiving member. The photoreceptor is then electrostatically charged uniformly in the dark and exposed through a red filter to an imagewise projection of a color image in register with said magenta developed image to form a second electrostatic latent image, which second image is developed with the complimentary cyan color toner and likewise transferred in register. The photoconductor is again electrostatically uniformly charged in the dark and then exposed through a blue filter to an imagewise projection of a color image in register with said magenta and cyan developed images to form a third electrostatic latent image which is then developed with the complimentary yellow toner and again transferred in register. The sequence of exposures through colored filters in this multiple development process may be performed in any suitable sequence other than the green, red and blue mentioned.
In these systems one important aspect resides in registration of the color toner image on the copy sheet, that is, the cyan, magenta, and yellow image should be in registration on the receiving member.
Generally, each developer used comprises a toner or resin color mixture in combination with different carriers. The toners used must posesss the appropriate color and continue to function under machine conditions which expose the developer to impaction and humidity among other undesirable factors. A three color system that has been well known and used in the past includes pigments of suitable cyan, magenta and yellow materials. Other disadvantages of the prior art processes include the requirements that, (1) the photoreceptor be panchromatic, (2) the development response of each of the three toner developers be constant with usage, (3) the transfer of the three different developed images be constant, and (4) the use of more than one different carrier material.
Also it is known in the prior art that three color layers can be coated one on top of the other, the first layer being the magenta layer, the second being the cyan layer and the third being the yellow layer. Each substractive color transmits two thirds of the spectrum and absorbs one third. The combination of cyan, magenta and yellow layers appears black, while the combination of magenta and yellow layers appears red, the combination of magenta and cyan layers appears blue, and the combination of yellow and cyan layers appears green.
More importantly in the prior art the colored toners must be combined with different carriers in order to obtain satisfactory images, such toners generally being negatively charged with respect to the carriers. For example, in one prior art process, the magenta toner is combined with an uncoated nickel berry carrier, the cyan toner is combined with a methyl terpolymer coated steel shot carrier, and the yellow toner is combined with an uncoated nickel berry carrier. Thus in such a system, at least two different carriers must be employed for the purpose of adjusting the triboelectric charges. Copying systems employing such developers require complex design elements and further make the system rather sensitive to outside influences because of the presence of two or three carriers and the charging relationship between the carriers and toner. In such systems, it is necessary to obtain approximately the same charge level and the same charge sign, on each toner, thus a unique carrier must be formulated for each toner. Also, the negative color developers are humidity sensitive, that is, they display high triboelectric charges at low relative humidity, and low triboelectric charges at high relative humidity. Thus, the development responses are not constant. Furthermore, each negative color developer has its own humidity-sensitivity since for example, the uncoated nickel berry carrier is more humidity-sensitive than the methyl terpolymer coated steel shot carrier. Additionally the negatively charged color toners are unsatisfactory for causing the development of images in a positively charged system, that is, where the photoreceptor, such as an organic photoreceptor is charged negatively.